The use of the component system of Si.sub.3 N.sub.4, SiO.sub.2, Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3 for making a silicon nitride comprising product is relatively new in the art. Such component system has been processed by hot pressing to obtain a densified product (see U.S. Pat. Nos. 3,969,125; 2,264,550; 4,264,548). But such component system has not been successfully processed to a densified product by sintering.
It is known that it is not possible to sinter the single component of Si.sub.3 N.sub.4 under ambient conditions. To achieve some degree of satisfactory densification, oxides must be added to the Si.sub.3 N.sub.4, extremely high temperatures must be employed (above the sublimation point of Si.sub.3 N.sub.4), and over-pressures as well as blankets must be used to contain the sublimation (see U.S. Pat. No. 4,285,895 by Mangels). This procedure does not achieve optimum economy and efficiency of processing. In an effort to eliminate the necessity for over-pressure during sintering, U.S. Pat. No. 4,354,990 discloses a method by which larger than usual amounts of oxides are added, while still using a packing blanket to contain sublimation at excessively high sintering temperatures. Again, this processing technique lacks economy and efficiency. Neither of these last two patents employ the Si.sub.3 N.sub.4 /Y.sub.2 O.sub.3 /SiO.sub.2 /Al.sub.2 O.sub.3 component system (which may afford enhanced strength characteristics) and thus fail to recognize the solubility enhancing characteristics of such system under sintering conditions.